This invention relates to the field of composite polymer films of reduced surface frictional characteristics and to a method of making such films. It more particularly refers to a biaxially oriented composite film structure having improved properties, including heat sealability, good machinability and reduced coefficient of friction.
In the packaging of certain types of foods, such as snack foods including candies, potato chips, cookies and the like, it is common practice to employ a multi-layer film. Polypropylene films are widely used in the packaging industry due to their superior physical properties, such as, transparency, stiffness, moisture barrier characteristics and others. Despite these highly desirable properties, unmodified polypropylene film has the disadvantageous property of having a high inherent coefficient of friction and film-to-film destructive blocking on storage. This high film-to-film coefficient of friction makes polypropylene films difficult to be successfully employed in automatic packaging equipment in their unmodified form.
In the past, coefficient of friction characteristics of polypropylene and other thermoplastic films have been beneficially modified by the inclusion in the polymer of fatty acid amides, such as, erucamide and oleamide. The effectiveness of this type of material depends upon its ability to migrate to the surface of the film in order to reduce the coefficient of friction. Both of the aforementioned amides are commonly used for reducing coefficient of friction. The development of the desired low coefficient of friction value is strongly dependent upon the type and amounts of amides, and time and temperature aging effects. Even the heat history of the film while in storage and shipping and during subsequent converter processes, significantly affects the coefficient of friction. In addition, the presence of these types of fatty acid amides on the film surface results in visible adverse appearance effects, manifested by an increase in haze, a decrease in gloss and the presence of streaks. These materials also adversely effect the wettability and adhesion of solvent and water-based inks, coatings and adhesives.
In the case of oriented polypropylene films which are widely used in the food packaging industry, it is common to laminate this film with itself or with other thermoplastic films or with paper films. When oleamide or erucamide are used in the polypropylene films a significant increase in coefficient of friction has been observed after lamination to such films. It is theorized that this is due either to the migration of the amide back into the polypropylene film or to the loss of the additive layer at the film surface. Therefore, these types of oriented laminated polypropylene films have limited usage for particular converting processes. Attempts to replace these amides to provide a consistent coefficient of friction have not been successful.
In U.S. Pat. No. 4,533,509, there is described a multi-layer structure comprising a comparatively thick base layer of an optically clear thermoplastic resin and a comparatively thin surface layer containing a finely divided inorganic material in a proportion sufficient to thereby impart anti-block characteristics and decrease film-to-film coefficient of friction. The resulting film has non-blocking and improved slip characteristics without the adverse appearance, wetting and adhesion effects typically experienced with amide-modified films. However, when laminated to other films, such structures exhibit significantly higher coefficient of friction values and cannot be used in conventional form, fill and seal machines.
U.S. Pat. No. 4,618,527, discloses a biaxially oriented film structure comprising (I) a comparatively thick base layer of a thermoplastic resin containing: an antistatic combination of (a) a compound of the formula RN(R1)2 wherein R is a C6-C24 alkyl group R1 is H(OCH2CH2)n; n=1-25; and (b) a monoester of an aliphatic C2-C24 fatty acid; and a slip improving proportion of (c) a C12-C24 fatty amide; and (II) a comparatively thin surface layer on at least one surface of (I), the surface layer comprising a thermoplastic resin containing an anti-block and COF reducing proportion of a finely divided inorganic material. This structure can be laminated to another film such as a polymeric film or a paper film.
In U.S. Pat. No. 4,652,489, a sealable, opaque polyolefinic multilayer film composed of a polypropylene base layer, a non-sealable surface layer, and a sealable surface layer, is disclosed. The. sealable surface layer has a low minimum sealing temperature and is made from a copolymer of propylene and ethylene or butene-1 units and/or a terpolymer of ethylene, propylene and butene-1 units. The non-sealable layer is a combination of a propylene homopolymer and a slip agent. The base layer contains an additive which is incompatible with polypropylene. The process includes stretching of the film, and during the stretching step, the polymer matrix is torn open around the additive particles to form vacuoles which give the base layer a degree of opacity
U.S. Pat. No. 4,659,612 discloses an oriented, polymer film laminate possessing a low coefficient of friction, good receptivity for water-based coatings and good optical characteristics.
U.S. Pat. No. 4,692,379 discloses an oriented, heat sealable polymer film laminate possessing a low coefficient of friction, good receptivity for water-based coatings and good optical clarity and a method for manufacturing such a laminate.
U.S. Pat. No. 4,720,420 discloses a sealable, transparent polyolefinic multilayer film composed of a polypropylene base layer, a non-sealable surface layer, and a sealable surface layer. The sealable surface layer has a low minimum sealing temperature and is made from a copolymer of propylene and ethylene or butene-1 units and/or a terpolymer of ethylene, propylene and butene-1 units. The non-sealable layer is a combination of a propylene homopolymer, a slip agent, and an anti-blocking agent. The process provided includes coextruding the three layers, biaxially stretching the coextruded film, and subjecting the non-sealable layer to a corona-discharge treatment.
U.S. Pat. No. 4,734,317 relates to a polyolefin film having a base layer composed essentially of propylene polymers and having sealing layers present on both sides, composed essentially of sealable olefin polymers. Polydialkylsiloxane is incorporated in a first of the two sealing layers, and this layer is not subjected to corona treatment. The second sealing layer is subjected to corona treatment and has polydialkylsiloxane, not incorporated, but present on its outer surface, the polydialkylsiloxane having been transferred to this layer by contact with the sealing layer in which polydialkylsiloxane is incorporated. The relative polydialkylsiloxane occupancy on the second layer, which is determined by means of ESCA spectroscopy, is not more than 15. The three-layer polyolefin film is prepared by a coextrusion process. It is particularly suitable for use as a packaging film on high-speed packaging machines.
U.S. Pat. No. 4,764,425 discloses an oriented, heat-sealable polypropylene film which possesses a low coefficient of friction, excellent anti-static behavior and good optical clarity.
U.S. Pat. No. 4,961,992 relates to a polymer film laminate having improved machinability on modern high speed belt drive machines, particularly when these machines are set up to form lap seals. A method of assembling such a film is also provided. The film includes an outside web having-an upper surface layer containing a silicone oil. This laminating web can be used with virtually any co-laminate, metallized or not, which is bonded thereto with an adhesive. Upon winding the composite film laminate upon a core, silicone oil is transferred to the inside surface of the laminates thus providing an inside coefficient friction which is about equal to or less than the outside coefficient of friction. Hot slip properties are also improved upon such transfer. The outside and inside webs are independently formed, which allows the inside web to include coatings or film layers which are not ordinarily usable in a silicone oil-containing film.
U.S. Pat. No. 5,110,671 discloses an oriented, multi-layer polymer film laminate having a core of isotactic polypropylene, an outside skin layer of isotactic polypropylene containing an anti-blocking agent and an inside heat sealable skin layer containing an anti-blocking agent and a coefficient of friction reducing amount of a silicone oil.
Despite these advances in the art, a need still exists for a film structure of improved machinability which also provides excellent sealability, strength and stiffness.
In accordance with the present invention, there is provided a film structure which includes at least one layer of an olefin homo-, co- or ter-polymer having a surface-treated external surface which is printable, sealable and machinable, the layer containing, as combined slip agent and antiblock, a particulate cross-linked hydrocarbyl-substituted polysiloxane. Particularly preferred particulate cross-linked hydrocarbyl-substituted polysiloxane include the polymonoalkylsiloxanes.
Also, a heat sealable multilayer film structure of improved machinability is provided. The film structure comprises an upper heat sealable layer comprising an olefinic homopolymer copolymer or terpolymer having a surface-treated external surface which is printable, sealable and machinable, the layer containing, as combined slip agent and antiblock, a particulate cross-linked hydrocarbyl-substituted polysiloxane, an intermediate layer comprising an olefinic polymer, and a lower heat sealable layer comprising an olefinic homopolymer, copolymer or terpolymer having an external surface which is sealable and machinable, the layer containing, as combined slip agent and antiblock, a particulate cross-linked hydrocarbyl-substituted polysiloxane.
A method of producing a-heat sealable multilayer film structure of improved machinability is also provided. The method includes the-step of coextruding a film structure, the film structure including an upper heat sealable layer comprising an olefinic homopolymer, copolymer or terpolymer having a surface-treated external surface which is printable, sealable and machinable, the layer containing, as combined slip agent and antiblock, a particulate cross-linked hydrocarbyl-substituted polysiloxane, an intermediate layer comprising an olefinic polymer, and a lower heat sealable layer comprising an olefinic homopolymer, copolymer or terpolymer having an external surface which is sealable and machinable, the layer containing, as combined slip agent and antiblock, a particulate cross-linked hydrocarbyl-substituted polysiloxane. In practicing the method of the present invention, it is particularly preferred to employ as the particulate dross-linked hydrocarbyl-substituted polysiloxane a polymonoalkylsiloxane.
By improved machinability is meant that the film exhibits a consistently low coefficient of friction and has improved anti-slip and non-blocking characteristics.
Accordingly, it is an object of the present invention to provide a film structure of excellent sealability.
It is another object of the present invention to provide a film structure with improved machinability.
It is a further object of the present invention to provide a film structure having an improved range of process operability.
It is yet another object of the present invention to provide a film which may be bonded to a wide variety of substrates and coatings.
It is a yet further object of the present invention to provide a film .structure in which a conventional treatment testing method can be used.
Other and additional objects, and the several advantages of the present invention, will become apparent to those of ordinary skill in this art upon a reading of the specification and the claims appended thereto.
In general, the present invention can be practiced with any thermoplastic film, for example, the polyolefins in general, and specifically, polymers, copolymers and blends of ethylene, propylene, butylene, etc., polyesters, such as, polyethylene terephthalate acrylic polymers and copolymers and the like. The basic film structures of the present invention find utility when supported or laminated to another film in order to accommodate a particular end use. Thus, polymer film layers different from the intermediate layer can be laminated to the intermediate layer of the structure for particular purposes. Such polymeric films include any thermoplastic film different from the intermediate layer of the film.
Particularly preferred polymers employed as the intermediate layer of the film herein are the polypropylenes which are highly isotactic. The preferred polypropylenes are well known in the art and are formed by polymerizing propylene in the presence of stereospecific catalysts system. They can have a melt index at 230xc2x0 C. ranging from about 0.1-25. The crystalline melting point is about 160xc2x0 C. The number average molecular weight ranges from about 25,000 to 100,000. The density ranges from about 0.90-0.91.
For descriptive purpose only, the film structures of the present invention will be described as having an upper skin layer (a), an intermediate layer (b) and a lower skin layer (c). As may be appreciated by those skilled in the art, the use of the terms upper and lower to refer to particular skin layers is merely relative. Moreover, although referred to as skin layers, the upper and lower layers may have additional structures bonded thereto, based on the functional requirements of the overall structure.
The polymer materials which are contemplated as the material for use in forming skin layers (a) and (c) are suitably exemplified by polyolefinic homopolymers, copolymers and terpolymers. Where homopolymers are to be used, they may be formed by polymerizing the respective monomer. This can be accomplished in a conventional manner by bulk or solution polymerization, as those of ordinary skill in this art will plainly understand. The copolymers are exemplified by and include block copolymers, for example of ethylene and propylene, random copolymers, for example of ethylene and propylene, and other ethylene homopolymers, copolymers, terpolymers, or blends thereof.
Preferred for use in forming upper skin layer (a) and lower skin layer (c) are heat sealable polymeric materials selected from the group consisting of ethylene-propylene-butene-1 (EPB) terpolymer, ethylene-propylene (EP) random copolymer, linear low density polyethylene (LLDPE), low density polyethylene (LDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE) or blends thereof.
Suitable EPB terpolymers are those obtained from the random inter-polymerization of from about 1 to about 8 weight percent ethylene, preferably from about 3 to about 6 weight percent ethylene with from about 65 to about 95 weight percent propylene, preferably from about 86 to about 93 weight percent propylene, butene-1 representing the balance. The foregoing EPB terpolymers are for the most part characterized by a melt index at 230xc2x0 C. of from about 2 to about 16 and advantageously from about 3 to about 7, a crystalline melting point of from about 100xc2x0 C. to about 140xc2x0 C., an average molecular weight of from about 25,000 to about 100,000 and a density within the range of from about 0.89 to about 0.92 gm/cm3.
The EP random copolymers generally contain from about 2 to about 7 weight percent ethylene, the balance being made up of propylene. The copolymers can have a melt index at 230xc2x0 C. generally ranging from about 2 to about 15 and preferably from about 3 to about 8. The crystalline melting point is usually from about 125xc2x0 C. to about 150xc2x0 C. and the number average molecular weight range is from about 25,000 to 100,000. The density will usually range from about 0.89 to about 0.92 gm/cm3.
In general, where blends of EPB terpolymer and EP random copolymer are used, such blends will contain from about 10 to about 90 weight percent EPB terpolymer and preferably from about 40 to about 60 weight percent EPB terpolymer, the balance being made up of EP random copolymer.
Heat sealable blends of homopolymer which can be utilized in providing layers (a) and/or (c) include about 1 to about 99 weight percent polypropylene homopolymer, e.g. one which is the same as, or different from, the polypropylene homopolymer constituting intermediate layer (a) blended with about 99 to about 1 weight percent of a linear low density polyethylene (LDPE).
Prior to extrusion, in accordance with the present invention, one or both skin layers (a) and/or (c) is compounded with an effective amount of a combined antiblocking and slip agent. Preferred combined antiblocking and slip agents are selected from the group of particulate cross-linked hydrocarbyl-substituted polysiloxanes. Particularly preferred are the particulate cross-linked polymonoalkylsiloxanes. Most particularly preferred are non-meltable polymonoalkylsiloxanes characterized as having a mean particle size of about 0.5 to about 20.0 um and a three dimensional structure of siloxane linkages. Such materials are commercially available from Toshiba Silicone Co., Ltd., worldwide, and in the United States from General Electric Co., and are marketed under the tradename Tospearl. Other commercial sources of similar suitable materials are also known to exist.
Although the polymer from which skin layer (a) is formed is not compounded with a silicone oil, this layer may ultimately acquire a machinability-improving and coefficient of friction-reducing amount of silicone oil. Thus, when the finished film laminate containing silicone oil on the exposed surface of skin layer (c) is taken up on a winding coil, some of the silicone oil will be transferred from this surface to the exposed surface of skin layer (a), primarily to the exposed surfaces of the combined antiblocking and slip agent particles which protrude from layer (a). However, since the interior of layer (a) contains no amount of silicone oil which could interfere with the heat sealing properties of this layer (and ordinarily contains no-silicone oil at all), the presence thereon of the transferred silicone oil serves to further enhance the coefficient of friction of the layer, improves its machinability, without significantly impairing its heat sealability.
Either or both layers (a) and (c) can also contain pigments, fillers, stabilizers, light protective agents or other suitable modifying ingredients if desired. Further, skin layers (a) and/or (c) can optionally contain a minor amount of an additional antiblock material, such as silica, clays, talc, glass, and the like. These antiblock materials can be-used alone, or different sizes and shapes can be blended to optimize machinability. The major proportion of these particles, for example, anywhere from more than half to as high as 90 weight percent or more, will be of such a size that a significant portion of their surface area, will extend beyond the exposed surface of such skin layer.
In order to enhance its receptivity for water-based coatings, the upper skin layer can be advantageously treated by flame treating.
Lower skin layer (c) of the film laminate can be of substantially the same composition as that of upper surface layer (a). In a preferred embodiment, the polymer constituting layer (c) is compounded with a silicone oil. The silicone oil advantageously possesses a viscosity of from about 350 to about 100,000 centistokes with 10,000 to about 60,000 centistokes being especially preferred. Examples of suitable silicone oils are polydialkylsiloxanes, polyalkylphenylsiloxanes, olefin-modified siloxane oils, polyether-modified silicone oils, olefin/-polyether-modified silicone oils, epoxy-modified silicone oils and alcohol modified silicone oils, polydialkylsiloxanes which preferably have from about 1 to about 4 carbon atoms in the alkyl group, in particular polydimethyl-siloxanes. Of the foregoing, the polydialkylsiloxanes, in particular, a polydimethylsiloxane, are preferred for use herein.
The silicone oil is added to layer (c) generally in the form of a dispersion or emulsion, the silicone being present within, as well as on the exposed surface of this layer as discrete microglobules, frequently of an average size of from about 1 to about 2 microns. The silicone oil, which is generally substantially uniformly distributed on the exposed surface of layer (c), is responsible for improving machinability and imparting a further reduction in the coefficient of friction to this surface as well as the exposed surface of layer (a) when some of the oil is transferred thereto after these surfaces have been placed in mutual contact, e.g., as will occur when the laminate film has been wound on a winding coil.
Polydimethylsiloxane or other silicone oils can be present at from about 0.15 to about 1.5 weight percent, preferably less than 0.5 weight percent, of lower layer (c). Some of this silicone oil will, of course, be present on the exposed surface of layer (c). The amount selected should in any event be sufficient to provide a coefficient of friction of layers (a) and (c) (following transfer of silicone oil microglobules to the latter) of about 0.4 or less, preferably between about 0.25 to about 0.3 up to at least about 60xc2x0 C. Because of the unique manner in which the silicone oil is applied to just the exposed surface of upper layer (a), such layer exhibits an improved coefficient of friction but not at the expense of its receptivity to water-based coatings, its heat sealability or its optical clarity.
The silicone oil should be incorporated as homogeneously possible in the polymer constituting layer (c). This can be achieved by either incorporating the silicone-oil as a dispersion or emulsion at room temperature and then heating the blend with the application of shearing forces or by incorporating the oil while the blend is being melted. The mixing temperature must be high enough to soften the blend and enable a very uniform mixture to be formed. The temperature required in a kneader or extruder is generally from about 170xc2x0 to about 270xc2x0 C.
Intermediate layer (b) can also contain anti-static agents, e.g., cocoamine or N,N bis(2-hydroxyethyl) sterylamine. Suitable amines include mono-, di, or tertiary amines.
Intermediate layer (b) will usually represent from about 70 to about 95 percent of the thickness of the overall film laminate or an even higher percentage thereof. Upper skin layer (a) and lower skin layer (c) are coextensively applied to each major surface of intermediate layer (b), usually by being coextruded directly thereon. For example, layers (a), (b) and (c) can be coextruded from a conventional extruder through a flat sheet die, the melt streams being combined in an adapter prior to being extruded from the die. Each of the skin layers (a) and (c) can comprise, for example, approximately 6.0% of the total thickness of the laminate. After leaving the die-orifice, the laminate structure is chilled and the quenched sheet then heated and stretched, e.g., five times in the machine direction (MD) and then subsequently, for example, eight times in the transverse direction (TD). The edges of the film can be trimmed. The film laminate is then wound on a reel in order to effect transfer of silicone oil from the exposed surface of layer (c) to the exposed surface of layer (a) as previously explained.
As a result of the biaxial orientation of the film structure herein, several physical properties of the composite layers,such as: flex-crack resistance, Elmendorff tear strength, elongation, tensile strength, impact strength and cold strength properties are improved. The resulting film can have, low water vapor transmission rate characteristics and low oxygen transmission rate characteristics. These improved physical properties make the film ideally suited for packaging food products, even those comprising liquids. The overall thickness of the laminate is not critical and advantageously can range from about 0.35 to about 2.0 mls.
Where the product of this invention is to be used as a label for goods, whether the label is opaque or transparent is a function of the objectives to be achieved. Where it is desired to hide the contents of the package being labeled, it would be preferable to use an opaque label. However, where it is more desirable to expose the contents of the package to customer examination, the label should be transparent.
A suitable peelable label stock is often put up as an assemblage of components. In one such assemblage, the components include: a base liner, a release layer disposed on the base liner, and a label stock, with a suitable pressure sensitive adhesive disposed thereon, releasably adhered to the release layer on the base liner. In instances where the adhesive is a pressure sensitive adhesive, a release layer is disposed on the base liner. In other cases the adhesive may be one activatable by other means, such as, heat, solvent, etc. Where the adhesive is not pressure sensitive, but is activatable by some other means, a release liner is not needed.
As indicated above, instead of a pressure sensitive adhesive, activatable adhesives can be employed for certain labeling techniques. For instance, when a label is to be applied about the full circumference of a package or bottle, water or other solvent can activate an adhesive stripe or strip applied to one end of the label. The label is then fixed in place by a slight overlap of the other end of the label. It has also be discovered that die cutting of labels is materially enhanced by including inorganic particles, such as calcium carbonate in the label skin layer close to the adhesive and/or release layer. These particles may also cause some cavitation as a result.
When an opaque label or film structure is desired, the core or intermediate layer of the film structure of the present invention may be formed in accordance with U.S. Pat. No. 4,377,616, the disclosure of which is incorporated herein by reference in its entirety for all that it discloses.
Where opacifying agents are desired, they may be incorporated in the intermediate composition of this invention, in a proportion of up to about 10%, preferably at least about 1%, by weight. Suitable conventional opacifying agents can be added to the melt mixture of the intermediate polymer before extrusion thereof into a film. Opacifying compounds are generally well known in this area. They may be exemplified by iron oxides, carbon black, aluminum, aluminum oxide, titanium dioxide, and talc.
The processability and machinability of the film may be further enhanced by the inclusion in the polymeric material used to form one or both skin layers of a small percentage of finely subdivided inorganic material. Such inorganic material not only can impart antiblock characteristics to the multi-layer film structure of the present invention, but also can reduce the coefficient of friction of the resultant film.
Contemplated finely divided inorganic materials, referred to above may be exemplified by: syloid, a synthetic amorphous silica gel, having a composition of about 99.7% SiO2; diatomaceous earth having a composition of, for example, 92% SiO2, 3.3% Al2O3, and 1.2% Fe2O3 which has an average particle size of about 5.5 microns, which particles are porous and irregularly shaped; dehydrated kaolinite (Kaopolite SF) having a composition of55% SiO2, 44% Al2O3 and 0.14% Fe2O3, which has an average particle size of about 0.7 microns, and which particles are thin flat platelets; and synthetic, precipitated silicates, for example Sipernat 44, a material having a composition of 42% SiO2, 36% Al2O3, and 22% Na2O, which has an average particle size of about 3-4 microns, and in which the particles are porous and irregularly shaped.
The polyolefin blends used to coextrude the multi-layer high opacity film structures contemplated herein are suitably formed by employing commercially available intensive mixers, such as those of the Bolling or Banbury type.
If desired, the exposed surface of skin layers (a) and/or (c) may have applied to it, coating compositions or substrates such as another polymer film or a laminate; a metal foil, such as aluminum foil; cellulosic webs, e.g. numerous varieties of paper, such as corrugated paperboard, craft paper, glassine, cartonboard, nonwoven tissue, e.g. spunbonded polyolefin fiber, melt-blown microfibers, etc. The application may employ a suitable adhesive, e.g. a hot melt adhesive, such as low density polyethylene, ethylene-methacrylate copolymer; a water-based adhesive such as polyvinylidene chloride latex, and the like.